1. Field of the Invention
The present invention relates to an on-demand offset printing device and printing method.
2. Description of the Related Art
Heretofore, a great number of printers employing an electrophotography system such as laser-beam printers, LED (light-emitting diode) printers, and so forth have been proposed. These printers have become widespread as on-demand printing applications. On the other hand, the planographic offset-printing method has been employed from ancient times as a productive application of a great amount of printed matter such as 1000 copies or more of printed matter having the same content, by using a plate.
Though improvement in image quality of printers employing the recent electrophotography system is remarkable, with printing wherein all over space is printed with one color, poor color uniformity (great color difference), curling, and so forth may occur. Also, printing as to thin paper such as with newspaper printing has not been realized.
On the other hand, the planographic offset-printing has become the mainstream of newspaper printing, and color printing has come to be able to be performed with thin paper, duplex, and high speed. Also, the planographic offset-printing has a principal feature in that the ink thickness on the printed matter is thin, such as 2 μm or so, so a printed matter can be finished without ruining the texture of paper.
However, with this printing, it takes time and cost for plate making, so a small number of output copies leads to increase in costs, and accordingly, this printing has not spread beyond a printing application of a great number of copies.
Therefore, there is demand for a new printing device in which the image quality of the planographic offset-printing method is obtained by on-demand while maintaining the simpleness of an electrophotography printer, and also realizing less cost and time even with a small number of output copies.
For example, with Japanese Patent Laid-Open No. 10-250027, a method for performing offset printing has been proposed wherein titanium oxide exhibiting hydrophobicity is formed on the surface of a latent image forming drum, and ultraviolet rays are irradiated on this according to the electronic data of an original to create a hydrophilic portion. Subsequently, after a “dampening solution” is held at this portion, ink is applied to the hydrophobic portion. Subsequently, the above hydrophilic portion returns to the original hydrophobicity by being heated to a predetermined temperature, and on-demand printing is enabled taking advantage of this process.
On the other hand, with Japanese Patent Laid-Open No. 11-291603, an on-demand relief printing method that does not employ this “dampening solution” has been proposed. First, an image is formed with ink-philic toner on the ink-repellent surface of a plate cylinder by the electrophotography method, and is temporarily fixed with heat. Subsequently, after ultraviolet cure waterless ink for offset is impressed on this toner, this ink image is hardened by being irradiated with ultraviolet rays, thereby forming a temporary “plate” which is firmer than the case of only a toner image. Waterless ink for planography is impressed on this plate again, and the ink thereof is transferred to a printing member (paper or the like). After ink supply and transfer operations are repeated a predetermined number of times, a toner image temporarily fixed on an object having the ink-repellent surface is removed along with an ink image left behind on the toner image, thereby enabling on-demand printing.
However, with the method disclosed in Japanese Patent Laid-Open No. 10-250027, unlike an offset plate having steps, a hydrophilic portion and a hydrophobic portion formed on a face having no steps are supplied with dampening solution and ink in an adhesion manner. Therefore, the edge (outline of a character or image) of ink which is a liquid is restrained only by dampening solution which is a liquid. As a result thereof, the edge of ink readily moves at the time of transferring ink to a blanket cylinder, and consequently, the outline of a character or image collapses from the original object to be formed, and printing quality is prevented from being kept in high quality, which is an issue.
Also, with the method disclosed in Japanese Patent Laid-Open No. 11-291603, ink to be printed is impressed on a protrusion made up of the temporarily fixed toner image (layer), and the layer where the ink impressed thereon is hardened by ultraviolet rays, and this ink is transferred to paper or the like. Therefore, the ink to be transferred to paper is put on the protrusion without any restraint from the surroundings, so the edge of the ink readily moves at the time of transferring the ink to paper. As a result thereof, the outline of a character or image collapses from the original object to be formed, and printing quality is prevented from being kept in high quality, which is an issue.
Also, a development process is passed through three processes of (1) Temporary fixing process by toner, (2) Application process of ink to be hardened as first coating, and (3) Application process of ink to be ultimately transferred to paper, so printing efficiency is poor, and there is room for improvement as to speeding up of printing speed.